The present invention relates to a technique for storage control, and in particular to a technique effectively applicable to a storage control unit or the like having a nonvolatile cache memory for temporarily storing user data.
Current market trends call for higher performance, larger capacity and lower cost storage units. In a redundant array of inexpensive disks (RAID), a plurality of disk units are configured in array. At the time data is written, a copy of the data is often written to a disk unit different from the one for storing the original write data. By doing so, in case of a fault in a disk unit in the array, the data in the defective disk unit can be “repaired” by the data in the redundant disk units, thereby improving reliability and maintainability of the disk units. With RAID, however, though the data reliability is improved, the operation of writing the redundant data increases the processing time, and therefore the write performance deteriorates.
For this reason, a write cache technique is indispensable for supporting RAID in the prior art. The write cache is mounted in the controller and the data are temporarily written to it. In response to the write request from the host, the data is written in this cache. Completion of the write operation is reported to the host at the same time that the data is written in the cache. Thus, the redundant data are generated, the write data and the redundant data are stored in the disk units asynchronously with the I/O processing for the host, thus preventing the deteriorated performance of the write operation.